As a blade, for instance, wind turbine blades of a wind power generator are exposed to flying objects, including hail, and thunderbolts in a natural environment. In addition, the tips of the wind turbine blades rotate at high speed, reaching 60 meters per second, and accordingly, the leading edges in particular are subject to the harshest of conditions. Conventionally, most wind turbine blades are made of fiber-reinforced plastic (FRP), especially glass-fiber-reinforced plastic (GFRP) that is made of an epoxy resin containing glass fibers, and the surfaces thereof are covered by a gel coating. However, the surface gel coating peels off due to flying objects and thunderbolts, and if rain water penetrates the blades through cracks in the blades created by the flying objects and thunderbolts, there is a possibility of the blades falling into disrepair in the worst case scenario. Even if such cracks are small, as a result of the blades being subject to a natural environment for a long period of time, degeneration caused by ultraviolet rays may develop if the epoxy resin is exposed through the cracks.